Northern Territory, NT, Australia
REE P U
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The Nolans Bore hydrothermal stockwork vein-style rare earth elements-phosphate-uranium deposit is located in the Reynolds Range, ~135 km NNW of Alice Springs in central Australia, within the Northern Territory (#Location: 22° 35' 37"S, 133° 14' 18"E).
The deposit is situated within the Aileron Province that occurs along the southern margin of the Northern Australian Element, and is dominantly composed of moderate to high grade 1.86 to 1.72 Ga metamorphosed psammitic rocks. During the 1.815 to 1.795 Ga Stafford, 1.790 to 1.745 Ga Yambah and 1.735 to 1.695 Ga events, this metasedimentary succession was intruded by widespread granitic and lesser mafic to ultramafic rocks (Scrimgeour, 2013).
Subsequently, during the 1.645 to 1.635 Ga Liebig and 1.590 to 1.560 Ga Chewings events, large sections of the Aileron Province was metamorphosed to high grade, amphibolite to granulite facies, accompanied by minor felsic and mafic magmatism (Scrimgeour et al., 2015; Vry et al., 1996; Williams et al., 1996; Rubatto et al., 2001). The high grade Liebig Event is interpreted to have accompanied the accretion of the Warumpi Provice to the south of the Aileron Province, and is restricted to the southern margin of the latter, and northern fringe of the former. The low pressure-high temperature Chewings event affected the central part of the Aileron Province that hosts the Nolans Bore deposit. This event is interpreted to be the result of a high heat flow regime, possibly an extensional back-arc basin setting (Korsch et al., 2011).
The Aileron Province rocks are overlain by Neoproterozoic to lower Palaeozoic successions of the Central Australian Superbasin. Both sequences were affected by the four phases of the Alice Springs Orogeny, distributed over the period from 450 to 320 Ma (Scrimgeour and Close, 2011).
In the Nolans Bore area, the 1.86 to 1.72 Ga metamorphosed psammitic rock sequence is represented by the Lander Rock Formation and Reynolds Range Group. The former is interpreted to be composed of two similar, but temporally discrete turbiditic suites, spanning the time intervals of 1.84 to 1.81 and 1.805 to 1.78 Ga, whilst the Reynolds Range group is inferred to have been deposited at ~1.785 Ga (Claoué-Long et al., 2008). In the deposit area, the Lander Rock Formation constitutes schist, phyllite, andalusite hornfels, garnet-cordierite-biotite-quartz granofels, sillimanite-biotite-cordierite-orthoclase granofels and tourmaline-bearing quartzite. The subsequent granitic intrusion, correlated with the 1806±4 Ma Boothby Gneiss and the Napperby Gneiss (1778±8 Ma; cited in Huston et al., 2011) underwent granulite facies metamorphism during the Chewings Event and associated late metamorphic pegmatite (Collins and Williams, 1995).
On a regional scale, within a 250 km radius, the Nolans Bore deposit and other REE occurrences are spatially and structurally associated with a series of tin and tantalum pegmatites, and the Mud Tank and Mordor Igneous Complex REE-bearing carbonatite/alkaline complexes. These occurrences are located in a region cut by three major lineaments defined by O'Driscoll (1985), including the NNW trending G2-gravity lineament (that also passes through the Olympic Dam IOCG deposit) and the NW trending G3-gravity and R16 lineaments (Hoatson et al., 2011).
At a district scale (over a 100 km interval) both the Mud Tank Carbonatite and Mordor Igneous Complex are located near the NW-SE deep-seated Woolanga Lineament, a crustal structure that may have influenced emplacement of alkaline rocks in the Aileron Province (Jaques, 2008; Hoatson et al., 2005). The mineral field also cuts across the broad WNW aligned northern Willowra Gravity Ridge high that straddles the boundary between the Aileron Province in the south and the Tanami Orogen in the north, interpreted to be a suture zone where the thickness of the Aileron Province crust increases to ~60 km (Goleby et al., 2009). The Nolans Bore deposit and other pegmatites, alkaline rocks and carbonatites in the area are therefore spatially related to a zone of thickened lithosphere crosscut by deep crustal structures (Hoatson et al., 2011).
The Nolans Bore deposit consists of a series of variably brecciated ENE trending, and steeply NW to NE dipping fluorapatite veins, mainly hosted by granitic gneiss of the Boothby Gneiss and to a lesser degree by Lander Rock Formation. There is also a small subset of sub-horizontal veins. The granite-gneiss host has been strongly kaolinised due to weathering, which has resulted in secondary enrichment of the REE.
Four styles of mineralisation have been recognised by Huston et al. (2011):
i). Massive fluorapatite veins that typically contain 4 to 6% REE oxides and make up most of the defined resource;
ii). Very high-grade (7 to 10% REE oxide) zones found in cheralite-bearing, apatite-poor kaolinitic zones external to the veins;
iii). Apatite-allanite-epidote zones hosted by calc-silicates; and
iv). Low-grade stockwork zones in gneiss and kaolinitised rock peripheral to the veins and mylonite zones.
The principal ore minerals are cheralite, thorite, allanite, bastnäsite, monazite and several REE-bearing fluorocarbonates (Lockyer and Brescianini, 2007; Huston et al. (2011). Brecciated apatite is often infilled and replaced with Ce- and La-bearing cheralite monazite and bastnäsite (Lockyer and Brescianini, 2007).
However, Hussey and Dean (2013) suggest, on the basis of more detailed drilling, the mineralisation at Nolans Bore occurs as a complex, three-dimensional vein system, that varies from continuous massive intervals of 4 to 7% REE mineralisation to lower-grade intervals, typically represented by a stockwork or cluster of veins, diluted by intervening unmineralised and variably altered country rock. Localised high-grade (up to about 40% REE) secondary mineralisation is also recognised, but only represents a small fraction of the identified Mineral Resources. The dominant mineralisation styles are massive fluorapatite, fluorapatite-allanite, fluorapatite-allanite-calcsilicate, plus kaolinitic- and clay-rich altered zones and equivalents, dominated by secondary monazite and crandallite group minerals, although numerous other minerals are known.
The deposit can be broadly divided into a North, Central and Southeast zone, distributed almost continuously within a 1200 x 900 m area, to depth of 250 m, each containing about a third of the defined Mineral Resource (Hussey and Dean, 2013).
Individual mineralised veins are up to tens of metres in width and can be hundreds of metres long. Mineralised vein margins vary from sharp to irregular, with most commonly showing adjacent calc-silicate alteration/wall rock replacement, which may be compositionally zoned, e.g., clinopyroxene to garnet. This calcsilicate alteration is variable, and is not always present, varying from very narrow in some places, to extensively developed in others. The variation in calc-silicate mineral assemblages appears to be largely related to the host rock composition. Early formed veins tend to be predominantly fine to very coarse fluorapatite infill containing fine-grained monazite and other REE mineral inclusions, sometimes locally overprinted by allanite-bearing veins. Weathering and oxidation complicates the mineralogy and textural features, and is strongly developed in the North Zone and the northern part of the Central Zone (Hussey and Dean, 2013).
As the Nolans Bore deposit occurs in a semi-regional zone that contains Carbonatite and alkaline igneous complexes, as well as tin- and tantalum-bearing pegmatites, Huston et al. (2011) suggest is possible that REE in the deposit were derived from alkaline and/or carbonatite melts or from felsic melts which formed tin-tantalum pegmatites. High thorium content of the Nolans Bore fluorapatite, led Hussey (2003) to suggest that it was unlikely to be related to a carbonatite or carbonatite-derived fluid, but is more likely to be sourced from a Niobium-Yttrium-Fluorine type (Cerny, 1991) pegmatite melt. The REE patterns for fluorapatite, however, are similar to that of apatite from both peralkaline and carbonatitic melts (Hussey, 2003). Juvenile initial 87Sr/86Sr of 0.705 to 0.707 and evolved Nd isotopes (Nd 1250 = -12 to -4) suggest an enriched mantle source of REE (Korsch et al., 2009).
The mineralised veins and breccias are hosted by granite gneisses interpreted to be emplaced at ~1.88 to 1.78 Ga, providing a maximum age for mineralisation. Preliminary dating of apatite from the veins yields an age of 1244±10 Ma (U-Pb; Korsch et al., 2009), which differs from the age of the Mordor Igneous Complex (SHRIMP U-Pb crystallisation age of 1133±5 Ma; Hoatson et al., 2005), nor with the 732±5 Ma Mud Tank Carbonatite (Black and Gulson, 1978). However, the ~1244 Ma age is broadly compatible with the a global carbonatitic and alkalic magmatic event between 1300 and 1130 Ma (Pidgeon, 1989). The REE mineralisation exhibits a thermal overprint related to the Alice Springs Orogeny (Hussey, 2003). REE in the zone of secondary enrichment indicate that primary REE mineralisation was remobilised during weathering and/or calcrete formation in the area.
Published JORC compliant mineral resources at October 2015 (Arafura Resources Limited website, viewed July 2016), using a cut-off grade of 1.0% TREO (Total Rare Earth Oxides), were:
Measured resource - 4.9 Mt @ 3.2% TREO, 13% P2O5, 0.245 kg/t U3O8;
Indicated resource - 30 Mt @ 2.7% TREO, 12% P2O5, 0.200 kg/t U3O8;
Inferred resource - 21 Mt @ 2.3% TREO, 10% P2O5, 0.163 kg/t U3O8;
TOTAL resource - 56 Mt @ 2.6% TREO, 12% P2O5, 0.191 kg/t U3O8.
For detail, consult the reference(s) listed below and "Hoatson D M, Jaireth S and Miezitis Y, 2011 - The major rare-earth-element deposits of Australia: geological setting, exploration, and resources; GeoScience Australia, 204p."
The most recent source geological information used to prepare this summary was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Hussey, K.J. and Dean, R.A., 2013 - New insights on the geology of the Nolans Bore rare earths deposit: in T.J. Munson and K.J. Johnston (Eds.) Northern Territory Geological Survey, 2013. Annual Geoscience Exploration Seminar (AGES) 2013. Record of abstracts. Northern Territory Geological Survey, Record 30013-001, pp. 77-80.|
Huston, D.L., Maas, R., Cross, A., Hussey, K.J., Mernagh, T.P., Fraser, G. and Champion, D.C., 2016 - The Nolans Bore rare-earth element-phosphorus-uranium mineral system: geology, origin and post-depositional modifications: in Mineralium Deposita v.51, pp. 797-822.|
Jaireth, S., Hoatson, D. and Miezitis, Y., 2014 - Geological setting and resources of the major rare-earth element deposits in Australia: in Ore Geology Reviews v.62 pp. 72-128.|
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